1. Field of the Invention
The present invention relates generally to analog function synthesizers, and particularly to a universal CMOS current mode analog function synthesizer that can realize over thirty-two nonlinear functions using a transistor squaring unit without dedicated current multipliers.
2. Description of the Related Art
The use of analog nonlinear networks, signal processing and medical equipment justifies the large number of analog nonlinear function synthesizers available in the literature. Over the years, different approaches have been used for synthesizing nonlinear functions. Initially, diodes and linear resistors were extensively used. Later on, the nonlinear characteristics of MOSFETs operated in weak or strong inversion regions and JFETs operated in the pinch-off or the triode region have been exploited in realizing various analog functions including the exponential, the squarer and the square rooting functions. Recently, the realization of nonlinear functions using piecewise-linear function approximations and integrated-circuit operational amplifiers, current conveyors, operational transconductance amplifiers and current comparators has been reported.
The exponential characteristic of the bipolar junction transistor has been exploited to advantage in the design of analog nonlinear functions using the translinear principle. Design of analog BiCMOS computational circuits has also been reported.
Over the years analog CMOS circuits have evolved based on the exponential law characteristic of a MOS transistor operating in weak inversion. Moreover, the square-law characteristic of a MOS transistor operating in strong inversion has been reported. Voltage multipliers, linear voltage-to-current and current-to-voltage converters, exponential and pseudo exponential current-to-voltage, voltage-to-current, voltage-to-voltage converters, vector-summation circuits, sin(x) shapers, square-rooters, arc sine function and arc cosine function are several examples of the analog nonlinear CMOS circuit realizations available in the literature. These circuit realizations suffer from many disadvantages. For example, the related art circuit realizations may permit only one function realization at a time. They may require numerical optimization routines to select the device size ratios and the bias voltages. They may use piecewise linear approximations for synthesizing the nonlinear functions. They may require programming of the parameters of several circuits. They may operate in voltage-mode with input and output voltages or mixed-mode with voltages as the input or output and current as output or input.
Current-mode circuits, with currents as input and output variables, are more attractive than their voltage-mode counterparts. This is attributed to wider signal bandwidths and larger dynamic ranges of operation that can be obtained using current-mode circuits rather than voltage-mode circuits.
Although a number of CMOS current-mode analog function synthesizers are available these circuits suffer from disadvantages such as, e.g., they may use piecewise linear approximations for synthesizing nonlinear functions; they may provide only a few functions (mostly the exponential or the pseudo exponential functions); they may extensively use integrated circuits such as operational transconductance amplifiers and current comparators; they may require digital control circuits to select the required function; they may realize only one function at a time.
Recently, a universal CMOS current-mode analog function synthesizer has been proposed. The key idea of the proposed circuit is the fact that numerous nonlinear functions can be approximated, to a high degree of accuracy, using a few terms of their Taylor series expansion.
Although a number of dedicated current multipliers are already available, current multipliers usually suffer from limited bandwidth, complexity leading to high power consumption, the need to trim out the feed-through terms (offset currents) and to adjust the scale factor (the multiplier gain).
It therefore would be desirable to present a new universal CMOS current-mode analog function synthesizer that can realize a wide range of nonlinear functions without recourse to dedicated current multipliers.
Thus, a universal CMOS current-mode analog function synthesizer solving the aforementioned problems is desired.